The present invention relates to a freezer in which articles to be refrigerated pass through the freezer on a porous belt. More particularly, the present invention relates to a cryogenic freezer in which the articles are refrigerated by nitrogen vapor being circulated through the belt and within the freezer. Even more particularly, the present invention relates to such a cryogenic freezer in which a bed of the article to be frozen is fluidized on the belt.
Industrial freezers incorporate a porous belt on which articles to be refrigerated are conducted through a freezing compartment from an inlet to an outlet of the freezing compartment. Various means are provided to produce refrigeration within the freezing compartment including the use of liquid and gaseous cryogens formed from liquefied carbon dioxide and nitrogen. The refrigeration is typically provided for cryogenic freezers by spraying a liquid cryogen into the freezing compartment through spray nozzles. Cryogenic vapor produced through the introduction of the liquid cryogen into the freezing compartment is circulated to refrigerate the articles. In a fluidized bed freezer, the cryogenic vapor is circulated with a sufficient velocity to fluidize a bed of articles to be frozen, passing through the freezer on the porous belt.
In prior art cryogenic freezers, some air enters the freezing compartment along with the articles to be frozen. The air contains moisture and such moisture freezes and accumulates on the belt as ice. Very often, food is frozen that has significant amount of moisture on its surface. Moisture released from the food will also enter the freezing compartment to accumulate on the belt as ice. In a fluidized bed freezer this is particularly troublesome in that the belt loses its porosity and therefore, the freezer loses its effectiveness.
Prior art methods for cleaning ice from the belt have included wire brushes to scrape off the ice and muting the belt outside of the freezer where a forced flow of ambient air is used to defrost the belt. These methods suffer from being either unreliable, overly complex, and/or thermally inefficient. Another method of belt cleaning in cryogenic freezers has been the introduction of externally vaporized cryogen into the freezer in the form of gas jets directed toward the belt. This method, however, is wasteful of the cryogen because the energy added to vaporize the cryogen represents wasted cooling potential.
As will be discussed, the present invention provides a cryogenic freezer of less complexity than the prior art and which conserves the cooling potential of the cryogen being used to freeze the articles.